A condenser microphone includes a condenser microphone unit (hereinafter, referred to as a “unit”), an impedance conversion circuit configured by a field effect transistor (FET), and an output connector. The unit includes a diaphragm and a fixed electrode disposed to face the diaphragm. The unit generates a voice signal based on a change in capacitance between the diaphragm and the fixed electrode.
The capacitance between the diaphragm and the fixed electrode is small, that is, several tens of μm, so that output impedance of the unit is extremely high. Thus, the unit outputs an output signal (voice signal) from the unit to an output connector via the impedance conversion circuit.
To cause the impedance conversion circuit to operate, power needs to be supplied to the condenser microphone. The power is supplied to the impedance conversion circuit, for example, using a phantom power supply system defined in JEITA RC-8162C “Power supply feeding for microphones”.
Generally, the FET is connected to the unit as a source follower circuit. In this case, the condenser microphone needs three lines (that is, a wiring system of three lines), which are a power supply line that supplies power to a drain of the FET, an output line that transmits a voice signal from a source of the FET, and a ground line (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-352622).
The condenser microphone is small and light weight, so that the condenser microphone can be used as a gooseneck microphone in a conference room and the like. The gooseneck microphone includes a first housing portion that accommodates the unit and an impedance converter, a second housing portion that accommodates an output connector, and a flexible pipe portion that connects the first housing portion with the second housing portion.
The three lines described above (the power supply line, the output line, and the ground line) are wired inside the flexible pipe portion. For example, if a light emitting diode (LED) is disposed in the first housing portion, a line for supplying power to the LED is also wired inside the flexible pipe portion. In this way, multiple lines are wired inside the flexible pipe portion, so that the flexible pipe portion tends to be large (thick).
A technique for reducing the number of lines connected to the unit and the impedance converter by employing a wiring system of two lines in which the power supply line and the output line are common has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2015-82676).
Generally, an output signal (voice signal) of the wiring system of two lines tends to be distorted more easily than the output signal of the wiring system of three lines, and a dynamic range of the wiring system of two lines is lower than a dynamic range of the wiring system of three lines. The technique disclosed in Japanese Unexamined Patent Application Publication No. 2015-82676 implements a high dynamic range even with the wiring system of two lines by including a collector grounding (emitter follower) transistor connected to the FET, and a resistor connected between a base of the transistor and the collector.